This invention relates to motion picture projectors and more particularly to an improved electric transport system for incrementally advancing film through a light gate in a motion picture projector.
Film projectors of a type commonly used in motion picture theaters typically comprise a high intensity light source, film supply and take-up reels and a drive system therefor, and a projection head including a light gate through which the film passes, a transport for incrementally advancing the film through the gate one frame at a time, a shutter and a lens system. The transport in the projection head often includes a single motor which drives both the shutter and a Geneva mechanism which periodically rotates a sprocket through a predetermined increment to advance the film through the gate. The Geneva mechanism is synchronized with the shutter through a gear drive such that the sprocket is rotated by an amount necessary to advance the film through the gate by one frame each time the shutter interrupts the projected picture. While a picture is projected, the Geneva mechanism inhibits movement of the sprocket to prevent any movement of the film in the gate.
Geneva mechanisms of a precision necessary for commercial motion picture projectors are quite expensive because of tight tolerance requirements. The mechanism must accurately advance the film in the gate to maintain a constant film alignment in the gate for each successive frame. If there is any variation in the alignment of successive frames in the light gate, the projected image will jump or jitter. When the image is projected on a large screen, such jitter is extremely annoying to the viewers and particularly to viewers nearest the screen. A Geneva mechanism has other disadvantages in addition to cost. A Geneva mechanism rotates the sprocket through predetermined fixed increments. Therefore, a projector must be designed for a specific film frame format and is not readily adaptable to other formats without replacing either the Geneva mechanism or the film sprocket, or both.
One solution to the problem of advancing different films by different increments for different frame sizes is suggested in U.S. Pat. No. 3,565,521 which issued Feb. 23, 1971 to Buttler et al. This patent suggests incrementally driving a servomotor to advance film through a light gate. The width of drive pulses to the servomotor may be changed for accommodating different picture frame sizes. The servomotor is stopped in response to a signal from an optical sensor detecting a hole in a positioning disc which is rotated with a sprocket which advances the film. However, it is possible for forces such as film tension to rotate the sprocket by a small amount in either direction from the position at which it is stopped. This looseness of the film advance sprocket may cause considerable jumping in the projected picture since any small movement or misalignment of the film in the light gate is greatly magnified as the projected picture is enlarged.
It has recently been desirable to automate motion picture theaters. Along these lines, some modern projectors have been designed to hold an entire show. By using two projectors, one projector can be controlled to automatically rewind the film from the take-up reel back through the projection head to the supply reel while the other projector is running with a second show. Ideally, the rewinding is at a much faster rate than the normal operating speed of the projector so that rewinding is completed before the second show is finished, even where the second show is shorter than the first show. However, a Geneva mechanism limits the rate at which the film can be rewound through the projection head since the Geneva mechanism intermittently moves the film when driven in a reverse direction as well as in the forward direction.